Musnad Ahmad 32:2126

'Affan narrated to us, Hammad narrated to us, from Thabit, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "There are three things, whoever possesses them will find the sweetness of faith: that Allah and His Messenger are more beloved to him than anything else besides them; that a man loves a man, not loving him except for the sake of Allah; and that a man would prefer to be cast into the fire than to return to Judaism or Christianity."